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INVESTMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale
Amortized Cost	$ 459,730	$ 676,129
Gross Unrealized Holding Gains	2	1
Gross Unrealized Holding Losses	224	831
Fair Value	459,508	675,299
Continuous Unrealized Loss Position less than 12 Months	224	831
Short-term | Commercial paper
Available-for-sale
Amortized Cost	52,595	81,026
Fair Value	52,595	81,026
Short-term | Certificates of deposit
Available-for-sale
Amortized Cost	26,068	11,869
Fair Value	26,068	11,869
Short-term | Municipal securities
Available-for-sale
Amortized Cost	127,149	469,604
Gross Unrealized Holding Gains	2	1
Gross Unrealized Holding Losses	82	740
Fair Value	127,069	468,865
Continuous Unrealized Loss Position less than 12 Months	82	740
Short-term | U.S. government agency securities
Available-for-sale
Amortized Cost	108,128	61,307
Gross Unrealized Holding Losses	95	88
Fair Value	108,033	61,219
Continuous Unrealized Loss Position less than 12 Months	95	88
Short-term | U.S. Treasuries
Available-for-sale
Amortized Cost	109,832
Gross Unrealized Holding Losses	45
Fair Value	109,787
Continuous Unrealized Loss Position less than 12 Months	45
Short-term | Variable rate demand notes
Available-for-sale
Amortized Cost	34,346	49,954
Fair Value	34,346	49,954
Long-term | Municipal securities
Available-for-sale
Amortized Cost	1,612
Gross Unrealized Holding Losses	2
Fair Value	1,610
Continuous Unrealized Loss Position less than 12 Months	$ 2
Long-term | U.S. government agency securities
Available-for-sale
Amortized Cost		2,369
Gross Unrealized Holding Losses		3
Fair Value		2,366
Continuous Unrealized Loss Position less than 12 Months		$ 3

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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